The present invention relates to a scanning electron microscope in which the dynamic range of luminous intensity of a display image is adjustable and an image signal processing method having the capability as above.
In recent years, the scanning electron microscope (hereinafter simply referred to as SEM) has been used widely and frequently in many industrial fields including manufacture of semiconductors. When applied to industrial use, the SEM is often used mainly in the inspection process. In the SEM used in the application as such, reduction of the difference in measurement characteristics among a plurality of SEM apparatus is particularly required.
When the same specimen is observed with a plurality of SEM apparatus, the luminous characteristics of displayed images, especially, their dynamic ranges need to be identical. But even if the detection sensitivity, for example, of a secondary electron detector in each of the plural SEM's is set identically, the detection sensitivity of the secondary electron detector or the amplification factor of an amplifier for detection signals slightly differs product by product and therefore, making the luminous dynamic ranges of display images uniform is difficult to achieve practically.
Luminous characteristics of display screen are often represented by a histogram indicating frequencies of or corresponding to luminous intensity levels of pixels of a display image (“luminous intensity” herein means “brightness of pixel” but it is indicated in a unit different from that for “brightness” ordinarily meant). For example, in JP-A-2005-174555 (paragraphs 0021 to 0043 and FIGS. 3 to 7), an example of an SEM is described in which the dynamic range of luminous intensity is adjusted by the amplification factor of a secondary electron detection signal amplifier circuit and besides, a histogram of luminous intensity of a display image is utilized for improving contrast or visibility.